Contamination Control in the Natural Gas Industry

By Thomas H. Wines and Saeid Mokhatab

Contamination Control in the Natural Gas Industry delivers the separation fundamentals and technology applications utilized by natural gas producers and processors. This reference covers principles and practices for better design and operation of a wide range of media, filters and systems to remove contaminants from liquids and gases, enabling gas industry professionals to fulfill diverse fluid purification requirements. Packed to cover practical technologies, diagnostics and troubleshooting methods, this book provides gas engineers and technologists with a critical first-ever reference geared to contamination control.

• Covers contamination control methods and equipment specific to the natural gas industry
• Includes guidelines on fundamentals and real-world technologies used today
• Gives engineers better design and operation with rating methods, standards and case histories

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 25, 2021
• ISBN: 9780128169872

Thomas H. Wines

Thomas H. Wines is currently a Director of Applications Development for Pall Corporation's Fluid Technologies and Asset Protection (FTAP) Group in Port Washington, NY. Prior to assuming the above role, Tom was a Director of Product Management, Senior Marketing Manager and Senior Staff Engineer for the Scientific and Laboratory Services Division at Pall. He has 31 years of filtration, separation, and purification experience serving the refinery, gas processing, and chemical industries, and is a specialist in the fields of liquid/gas and liquid/liquid coalescing. He has authored over 50 technical publications and given numerous presentations in this field at professional societies. He holds a PhD in Chemical Engineering from Columbia University, and is a member of AIChE.

Book preview

Contamination Control in the Natural Gas Industry

Thomas H. Wines

Pall Corporation, United States

Saeid Mokhatab

Gas Processing Consultant, Canada

Table of Contents

Cover image

Title page

Copyright

Dedication

About the authors

Preface

Acknowledgments

Disclaimer

1. Fundamentals of filtration science

Abstract

Chapter Outline

1.1 Introduction

1.2 Overview

1.3 Darcy’s law

1.4 Capture mechanisms

1.5 Filter life

1.6 Differences between solid–liquid and solid–gas separation

References

2. Fundamentals of separation science

Abstract

Chapter Outline

2.1 Introduction

2.2 Liquid/gas systems

2.3 Liquid/liquid systems

References

Further reading

3. Industrial contaminants

Abstract

Chapter Outline

3.1 Introduction

3.2 Origins and types

3.3 Characterizing contaminants

References

Further reading

4. Industrial filtration technologies

Abstract

Chapter Outline

4.1 Introduction

4.2 Gravity separators

4.3 Basket strainers

4.4 Filter press

4.5 Cyclonic separators and cyclo-filters

4.6 Disposable cartridge filters

4.7 Regenerable filters

4.8 Other filtration technologies

4.9 Filtration summary

References

Further reading

5. Industrial separation technologies

Abstract

Chapter Outline

5.1 Introduction

5.2 Liquid/liquid separation

5.3 Liquid-gas separations

5.4 Three-phase separations

References

6. Role of chemical additives

Abstract

Chapter Outline

6.1 Introduction

6.2 Surfactants

6.3 Typical chemical additives

6.4 Process applications

6.5 Effect on filtration/separation

References

Further reading

7. Effect of contamination on processes in the natural gas industry

Abstract

Chapter Outline

7.1 Introduction

7.2 Natural gas supply chain

7.3 Gas production at well head

7.4 Gas processing plant

7.5 Pipeline

7.6 Underground storage

7.7 Liquefied natural gas production

References

Further reading

8. Diagnostics and troubleshooting methods

Abstract

Chapter Outline

8.1 Introduction

8.2 Strategic approach

8.3 Field methods

8.4 Lab methods

8.5 Applications/case studies

References

9. Filtration and separation rating

Abstract

Chapter Outline

9.1 Introduction

9.2 Solid/liquid filter rating standards

9.3 Liquid/liquid separation rating standards

9.4 Solid/gas separation rating standards

9.5 Liquid/gas separation rating standards

9.6 Filter and coalescer characterization methods

References

Appendix 1. Conversion factors

Appendix 2. Cartridge diameter factors

Appendix 3. Carbon steel pressure vessel and nozzle diameters

Index

Copyright

Gulf Professional Publishing is an imprint of Elsevier

50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States

The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, United Kingdom

Copyright © 2022 Elsevier Inc. All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

Library of Congress Cataloging-in-Publication Data

A catalog record for this book is available from the Library of Congress

ISBN: 978-0-12-816986-5

For Information on all Gulf Professional Publishing publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Joe Hayton

Acquisitions Editor: Katie Hammon

Editorial Project Manager: Tracy I. Tufaga

Production Project Manager: Sojan P. Pazhayattil

Cover Designer: Christian J. Bilbow

Typeset by MPS Limited, Chennai, India

Dedication

This book is dedicated to my family which in these times has become even more significant than ever and especially to my wife, Donna, and children, Daniel and Shannon, for their pandemic support and patience with this project over the last 2½ years.

—Thomas H. Wines

I wish to dedicate this book to my wife, Maryam, and my son, Sam, whose patience, understanding, and encouragement were essential to completion of this book. They contributed to this effort in ways that I probably will never know or understand.

—Saeid Mokhatab

About the authors

Thomas H. Wines is currently a Director of Applications Development for Pall Corporation's Fluid Technologies and Asset Protection (FTAP) Group in Port Washington, NY, United States. Prior to assuming the above role, he was a Director of Product Management, Senior Marketing Manager, and Senior Staff Engineer for the Scientific and Laboratory Services Division at Pall. He has 33 years of experience in filtration, separation, and purification serving the refinery, gas processing, and chemical industries, and is a specialist in the fields of liquid–gas and liquid–liquid coalescing. He has authored over 50 technical publications and given numerous presentations in this field at professional societies. He holds a PhD in Chemical Engineering from Columbia University, and is a member of AIChE.

Saeid Mokhatab is recognized globally as a process technology expert in the fields of natural gas processing and liquefied natural gas (LNG). For over two decades, he has been actively involved in different aspects of several large-scale gas processing and LNG projects, from conceptual design through plant startup and operations support. He has contributed to the understanding of gas processing and LNG knowledge, practices, and technologies through 300 technical papers and three reference books (published by Elsevier in the US), which are widely read and highly respected. He founded the Elsevier peer-reviewed Journal of Natural Gas Science and Engineering and has served on the editorial/advisory boards of leading professional publications pertaining to gas processing and LNG. He has also been an active member of several professional societies, including the Society of Petroleum Engineers (SPE) and Gas Processors Association (GPA) Europe, and has served on the technical program/advisory committee of many acclaimed gas processing conferences worldwide. As a result of his outstanding work in the natural gas industry, he has received a number of international awards and medals and has been listed in highly prestigious biographical directories.

Preface

Thomas H. Wines and Saeid Mokhatab

Natural gas revolutionized the global energy market over the last 20 years with the development of horizontal fracking of shale formations starting in the United States. This led to a boom in production that was so successful that it reduced the price of natural gas significantly and caused its own downfall with the crash of the gas production market. A second wave of production grew out of the need for chemical feedstocks from wet natural gas wells that contained significant natural gas liquids. A third wave is expected for the increase in liquefied natural gas (LNG) production to feed an energy hungry world.

The overall natural gas industry encompasses production from the wellhead sites, purification of the gas in processing plants, extensive transmission pipeline networks and ultimately liquefaction to make LNG. A multitude of process operations have been developed to accomplish these steps and the need for contaminant control is evident throughout these processes. In fact, a commerce sector has flourished due to contaminant control providing vital separation equipment, consulting, and application troubleshooting services. The subject of contaminant control, however, is usually overlooked in University courses and is mostly learned on the job and in many cases only after painful lessons by natural gas engineers.

Much of the information related to contaminant control is scattered across various journal articles and may not be readily accessible. It was the authors’ intent to consolidate this information into one source for both convenience and continuity so that various aspects of this subject area could be compared, to consider their benefits and potential limitations. In writing this work, the authors strived to provide practical and useful information for both the novice and the more experienced, providing more scientific depth in certain areas and sufficient technical references if the reader is so inclined to dive in deeper. One of the most difficult aspects of preparing this book was on how to balance the content between the overview of many topics and when and how far to delve into the scientific details.

A passion shared by both authors is the ability to diagnose, troubleshoot, and provide solutions to problems that arise in the natural gas industry. It is our hope that the information provided in this book on contamination control will provide the information needed to assist the reader for these purposes. The authors will be well served if this is accomplished and grateful if the readers find the materials interesting and of practical use.

Acknowledgments

An invaluable contribution to this book is the insight by experts in their specialties and applications. Special thanks are due to friends and colleagues, who encouraged, assessed, and made this book possible. Dr. Thomas H. Wines also expresses his sincere gratitude to some people at Pall Corporation that were invaluable in completing this work. Among them are Sean Meenan, Senior Vice President-FTAP Group, for his encouragement and allowing him the space to pursue this project in a demanding work environment; Michael Forzano, Chief IP Consul, for expediting the legal contract that preceded this project’s initiation; Suzanne Hennings, Librarian-Resource Service, who provided excellent support in helping him navigate the research search engines and obtain permissions for use of figures and tables; Paul Jones, Director-Quality Assurance and Regulatory, for his quality and assurance reviews; and a special thanks to Dr. Ali Arshad, Senior Director-FTAP Group, for his time consuming, meticulous technical review and comments on the manuscript.

We deeply acknowledge the greatest help of Mr. Cris Heijckers and Mr. Guy Hellinx of Kranji Solutions Pte Ltd., Singapore, who prepared two sections on Strategic Approach and Case Histories in Chapter 8 (Diagnostics and Troubleshooting Methods). We also appreciate the editorial staff members of Elsevier who have been an excellent source of strong support during the preparation and publication of this book.

Disclaimer

This book is intended to be a learning tool. The materials discussed in this book are presented solely for educational purposes and are not intended to constitute design specifications or operating procedures. While every effort has been made to present current and accurate information, the authors assume no liability whatsoever for any loss or damage resulting from using them.

All rights reserved. This book is sold subject to the condition that it shall not by way of trade or otherwise be resold, lent, hired out, stored in a retrieval system, reproduced or translated into a machine language, or otherwise circulated in any form of binding or cover, other than that in which it is published, without the prior written permission of the authors and without a similar requirement including these conditions being imposed on the subsequent purchaser.

1

Fundamentals of filtration science

Abstract

Many factors go into deciding the type of separation equipment that should be used for a given application. Having a basic understanding of how the separation is accomplished and what options are available are important to make the best choices to optimize plant operation. The objectives of this chapter are to introduce the most commonly found filtration and separation technology options used in the oil and gas industry and provide insights into the fundamental governing theory on how they work to lay a framework for further discussion.

Keywords

Filtration; separation; liquid; fibers; solids; applications

Chapter Outline

Outline

1.1 Introduction 1

1.2 Overview 1

1.2.1 Major types of separation equipment used to treat gas streams 2

1.2.2 Major types of separation equipment used to treat liquid streams 7

1.3 Darcy’s law 10

1.4 Capture mechanisms 13

1.4.1 Direct interception (sieving) 13

1.4.2 Inertial impaction 14

1.4.3 Diffusive capture 15

1.5 Filter life 16

1.5.1 Filter type 18

1.5.2 Void volume 18

1.5.3 Flux 19

1.6 Differences between solid–liquid and solid–gas separation 20

References 21

1.1 Introduction

Many factors go into deciding the type of separation equipment that should be used for a given application. Having a basic understanding of how the separation is accomplished and what options are available are important to make the best choices to optimize plant operation. The objectives of this chapter are to introduce the most commonly found filtration technology options used in the oil and gas industry and provide insights into the fundamental governing theory on how they work to lay a framework for further discussion.

1.2 Overview

The removal of contaminants in the oil and gas industry will generally fall into four categories: solids from liquid, solids from gas, liquids from gas, and liquids from liquids.

Filtration, separation and many of the terms used in the industry such as coalescer can have different meanings which can often time lead to confusion. The following conventions will be used for this book for the sake of clarity and to provide the reader with an overview of the various technologies available. The separations here are purely mechanical and removal of contaminants that are dissolved by any type of adsorption or by reaction are not covered. The major types of separation equipment used to treat gas and liquid streams along with commentary on the type of separation, separation mechanism, size removal, benefits, and drawbacks are described below. A brief overview of the separation equipment including the relative expenses are also given in Tables 1.1 and 1.2 for gas and liquid streams, respectively. In many cases, an industrial application may use more than one of these technologies to achieve a desired separation and to meet cost goals.

Table 1.1

Table 1.2

1.2.1 Major types of separation equipment used to treat gas streams

Knock out: Consists of a vessel with no internals that induces an expansion and resultant reduction in the velocity of the gas allowing for liquids to disengage from the flow and be collected at the bottom of the vessel. Knock outs are designed to catch slugs of liquids typically with drops 300 μm and larger. They are often used as a first stage separator followed by more efficient devices. Generally, knock outs are not affected by high solids and the separation mechanism is gravity. At reduced flow rates, they will exhibit improved separation.

Cyclone: A device that directs the gas flow in a circular motion to cause liquids to impact on the walls of a tube by centrifugal force. The liquids then drain downward by gravity to be removed. Cyclones can be made up of one single unit (vessel) or multiple smaller tubes that fit inside a vessel. Typically, cyclones will have good removal efficiency for drops 10 μm and larger with multiple tubes. Cyclones are used for liquid removal from gas and not affected by high solids levels. As the velocity of the gas swirling in the tubes increases, the separation efficiency and pressure drop will correspondingly increase, and the converse is true so that at reduced flow rates, cyclonic separators will lose efficiency.

Gas particle filter (disposable): Consists of a disposable cartridge type filter that can have a number of configurations including: a pleated filter media wrapped around a metal core (out-to-in flow), a melt blown depth media wrapped around a metal core (out to in-flow), and a pleated media coreless pack that fits into a permanent porous metal basket for support (in-to-out flow). Filter media can be different type of polymers including polyester, polypropylene, nylon, polyphenylene sulfide or can be made of other materials such as glass fiber or cellulose.

The gas flows through the filter media and solids are removed until the pressure drop increases to the point where the filter cartridge is changed. Gas particle filters come in different efficiency ratings based on the type of media used and will span the range of 1-100 μm offering very high removal efficiencies for the rated micron size and above. Generally, they are used when solid contaminants are mid-to-low level concentrations as high solids can cause short service life and excessive change outs leading to poor economics. The separation mechanism is direct interception and diffusive capture. It is not affected by flow reduction and will either maintain or increase separation efficiency because of lowered flowrate.

Blowback gas filter (re-generable): Consists of a permanent cartridge type filter that is either constructed of metal or ceramic media. The gas flows through the filter media from the out to the in direction and solids form a porous cake on the outside of the filter. At a set differential pressure, a pulse of pressurized gas is directed in the reverse flow direction to dislodge the collected solids that are then collected in a bottom chamber. The normal gas flow is then established, and the solids start to form another cake layer and the process is repeated. Blowback filters can last for multiple years before requiring a chemical cleaning. They may last for decades before requiring replacement.

Blowback filters will use filter medium that is in the few micron range in gas service. The filter media should not be exceedingly coarse as this would lead to plugging by the solids and a finer pore structure will allow these solids to form a cake and stay on the exterior of the media for long service life. Blowback applications are limited to processes that have solids that will form a cake (gasification/catalyst recovery) and usually where there are no sticky materials or liquids present. Typically used for applications at high temperatures. Blowback filters require a more complex system that includes gas nozzles and automated valve sequencing to create the back pulses and also will require a separate vessel or accumulator for the high-pressure gas.

The Blowback system will be inherently costlier for capital expenditure than disposable gas particle filters but will operate in harsher environments and economically at higher solids challenges to the filters. The separation mechanisms are direct interception, inertial impaction, and diffusive capture. It is not affected by flow reduction and will either maintain or increase separation efficiency as a consequence of lowered flowrate.

Vane pack: Contains a series of parallel solid sheets with frequent bends that cause the gas flow to change direction repeatedly. Mostly constructed from metals but can also be made of plastics for compatibility reasons. Designed for liquid separation from gas and as the gas changes direction, any liquid drops with sufficient inertia will impact the vane walls and be separated from the gas flow. The liquids that impact on the vane wall will drain downward by gravity and be collected in a lower sump.

Normally, will have good removal efficiency for drops 10 μm and larger. Not affected by high levels of solids and the separation mechanism is inertial impaction. At reduced flow rates, it will exhibit lower separation efficiency as the liquid drops will have reduced momentum and are less likely to leave the gas streamlines and impact onto the vane walls.

Mesh pad: Comprised of a mat or pad of intertwined fibers that is several inches thick and can be made in various geometries mostly circular or rectangular. As the gas passes through the mesh pad, liquid drops will leave the gas streamlines and impact onto the mesh fibers. Once on the mesh fibers, the drops will coalescer or merge together to create larger drops that will settle by gravity. Mostly constructed from metals but can also be made of plastics for compatibility reasons. Designed for liquid separation from gas streams.

Usually, will have good removal efficiency for drops 10 μm and larger. Not affected by high levels of solids and the separation mechanism is inertial impaction. At reduced flow rates, the mesh pad will exhibit lower separation efficiency as the liquid drops will have reduced momentum and are less likely to leave the gas streamlines and impact onto the mesh fibers.

Filter-sep: Utilizes a two-stage approach to separation in a horizontal configured vessel. The gas first passes through cartridge filters where fine aerosol drops coalesce into larger drops and solids are captured. The larger drops leaving the filter cartridges then are removed by either a mesh pad or a vane pack in a second stage per the same way previously described for these separation technologies that have the primary separation mechanism of inertial impaction.

Usually, will have good removal efficiency for drops 10 μm and larger. Not able to process slugs of contaminants very well as the inlet gas goes directly to the filter cartridges. At reduced flow rates, the filter-sep will exhibit lower separation efficiency as the liquid drops will have reduced momentum and are less likely to leave the gas streamlines and impact onto the second stage mesh pad or vane pack.

Liquid–gas coalescer: Consist of a specially designed cartridge with medium that has a graded pore size. The inlet section contains the smallest pores to increase capture efficiency of small aerosol droplets. As the gas flows through the media, the pore size increases allowing the small drops to combine or coalesce into larger more easily separated drops. The diameter of the fibers is much smaller than the mesh pad as well as the spacing between the fibers to allow for increased separation efficiency of finer drops.

Typically, will have good removal efficiency for fine drops of 0.3 μm and larger. The separation mechanisms are a combination of direct interception, inertial impaction, and diffusive capture. At reduced flow rates, the liquid–gas coalescer will exhibit the same or improved separation efficiency.

1.2.2 Major types of separation equipment used to treat liquid streams

Decanter: Consists of a vessel with no internals that allows for a reduction in the velocity of the two phase inlet liquid stream so that the two phases of liquids have sufficient residence time to separate. Depending on the type of liquid that is the main or continuous phase and what type of liquid is the minor or discontinuous phase, a collection sump is located on the top (hydrocarbon from water) or on the bottom (water from hydrocarbon). Decanters operate as bulk separators, typically removing drops 150 μm and larger. Often, it is used as a first stage separator followed by more efficient devices. Generally, not affected by high solids and the separation mechanism is gravity. At reduced flow rates, it will exhibit the same or improved separation.

Plate separator: Contains a series of parallel plates that are inclined (commonly 45 degree or 60 degrees) to reduce the settling zone that the dispersed drops need to travel. The spacing between the plates is typically in the range of an inch or less. It is constructed from metal or plastic depending on compatibility needs. It is intended for liquid–liquid separation. The dispersed drops that impact on the plate walls will drain downward by gravity and be collected in a lower sump or by buoyancy and rise to a top sump.

It is usually designed for removal of drops 50 μm and larger and is not affected by high levels of solids. The separation mechanism is either gravity/buoyancy settling. Separation performance will improve at reduced flow rates.

Mesh pad: Similar in construction to the mesh pad used in liquid–gas separation applying a thick fibrous bed of several inches in thickness. The discontinuous drops are carried by the bulk flow through the mesh pad and leave the flow streamlines to impact onto the mesh fibers. Once on the fibers, the small drops will coalesce or merge to become larger, more easily separated drops. The larger drops leave the mesh pad and are separated downward by gravity and collected in a bottom sump when heavier than the bulk phase (water from hydrocarbon) or when the drops are lighter than the main phase (hydrocarbon from water) will rise due to buoyancy and be separated in a top sump. It is designed for liquid–liquid separations.

They are typically used for less critical applications and for easy-to-separate emulsions with larger drops sizes and exhibit good separation efficiency for drops over 100 μm. They are not sensitive to fouling by high solids levels. The primary separation mechanism is inertial impaction and will lose efficiency at reduced flow rates due to lower inertia of the discontinuous phase drops.

Media bed: Consists of a vessel filled with granular solids. Different material can be used and common types are sand, anthracite, activated carbon, and garnet. The bed creates a tortuous path for the flow and the main mechanisms for separation are inertial impaction and direct interception. The granular bed can be made up of multiple layers of different materials and of different porosity. It is used for both liquid–liquid separation and for solids removal. When the bed becomes plugged with solid contaminants, it is regenerated through a backwash that involves flowing in the reverse direction to flush out accumulated solids that are collected in a waste stream. Depending on the size of the solid grains, the solids or drop removal size can be as low as 10 μm. The use of activated carbon can improve separation by adding in adsorption capabilities to remove dissolved contaminants.

Centrifugal separator: A device that directs the gas flow in a circular motion to cause liquids to impact the walls of a tube by centrifugal force. The liquids then drain downward by gravity to be removed. Usually it uses the term Cyclone when the swirling motion is created from the flow in the pipe without any external forces. The term Centrifuge is used when the separation device includes a motorized pump to create the rotating fluid motion. Cyclones can be made up of one single unit (vessel) or multiple smaller tubes that fit inside a vessel. They will typically have good removal efficiency for drops 10 μm and larger with multiple tubes. They are used for liquid–liquid removal and not affected by high solids levels. As the velocity of the gas swirling in the tubes increases, the separation efficiency and pressure drop will correspondingly increase, and the converse is true so that at reduced flow rates, cyclonic separators will lose efficiency.

Electrostatic coalescer: Entails a liquid–liquid separation system that uses charged electrodes in a low conductive continuous fluid (hydrocarbon) to separate an aqueous dispersed phase. The separation mechanism is based on charge separation as the aqueous drops will exhibit a negative or positive charge that causes them to migrate to the electrodes and coalesce with other drops. The coalesced drops will drain to the bottom of the vessel due to gravity and the purified hydrocarbon will exit the top of the vessel. It is used for liquid–liquid separations. It is not affected by high solids and will remove drops greater than 10 μm.

Crossflow filter: Comprised of a filtration system that utilizes tangential flow so that the majority of the flow passes by the filter medium in a recirculating stream and a smaller portion actually passes though the filter medium and is purified. Used for high solids content and for fouling type solids (gels, sticky materials). Typically uses a membrane filter medium that is rated between 0.1 and 1 μm and separation mechanism is direct interception. Usually the membrane is periodically chemically cleaned and re-used.

Filter press: Made up of a filter system that uses a series of porous cloth spacers that are preconditioned using a slurry of a filter aid, diatomaceous earth (DE) most commonly used, to create a series of filter cake surfaces. These filter surfaces are pressed together and mechanically held in place during the filtration stage. Once the filter has been plugged with solids, the press is loosened, and the fouled filter aid is removed and becomes waste. The process is then repeated by preconditioning the porous clothes again to form the filter cake surfaces and filtration is resumed. The separation mechanism is direct impaction and this type of filter can handle high solids levels. The separation removal is mostly limited to contaminants >5 μm and is used for solids from liquid separation.

Liquid particle filter (disposable): Similar in construction to the gas particle filter that contain a filter medium (pleated or depth) wrapped around support core (can be integral or a separate permanent core) for out-to-in flow or for in-to-out flow can use a permanent porous metal basket for support. A number of different materials are applied for the filter media including polymers (polyester, polypropylene, nylon, polyphenylene sulfide) as well as other materials such as glass fiber or cellulose.

The liquid containing suspended solids enter the filter cartridges where the solids are captured onto the filter medium and the liquid continues downstream. As the solids are collected, the pores in the filter medium become plugged until the differential pressure across the filter cartridges increases up to the designated change-out differential pressure. Cartridge filters are available in a wide range of size removal ratings from 1 to 100 μm. Efficiency will vary depending on whether the filter cartridges are Nominally or Absolute rated. Liquid cartridge filters will have a service life that will range from weeks to months depending on the total suspended solids levels. Generally, it is only feasible for economics when the solid levels are <50 ppm at which point backwash filters are preferred. It operates on the capture mechanism of direct interception, and as a consequence the separation efficiency is not affected by reduced flow rates.

Backwash filter (re-generable): Consists of a permanent cartridge type filter that is either constructed of metal or polymeric media. The liquid flows through the filter media from the out-to-in direction and solids form a porous cake on the outside of the filter. At a set differential pressure, a pulse of clean pressurized liquid is directed in the reverse flow direction to dislodge the collected solids that are then collected in a bottom chamber. The normal liquid flow is then established, and the solids start to form another cake layer and the process is repeated. Backwash filters can last for several months to multiple years before requiring a chemical cleaning. They may last for decades before requiring replacement.

Backwash filters will use filter medium that is usually in the 1-20 μm range. Separation mechanism is primarily direct interception. The filter media should not be too coarse as this would lead to plugging by the solids and a finer pore structure will allow these solids to form a cake and stay on the exterior of the media for long service life. Backwash applications are limited to processes that have solids that will form a cake and usually where there are no sticky materials such as tars or gels present. Backwash filters require a more complex system that includes liquid nozzles and automated valve sequencing to create the back pulses and also will require a separate vessel or accumulator for the clean high-pressure reverse flow liquid supply.

The Backwash filter system will be inherently costlier for capital expenditure than disposable liquid particle filters but will operate in harsher environments and economically at higher solids challenges to the filters. Backwash filters are not affected by flow reduction and will either maintain or increase separation efficiency as a consequence of lowered flowrate.

Liquid–liquid coalescer: Constructed using a special medium designed for coalescing, whereby the pores at the inlet are fine and as the flow passes through the coalescer medium, the pores become coarser to facilitate the growth or coalescence of drops in the discontinuous phase. The discontinuous phase drops in the inlet can be in the micron size range and are transformed to drops in the millimeter size range in the outlet making them easier to separate. Vertical liquid–liquid coalescers can be applied to water from hydrocarbon separation and will usually have a second stage separator that consists of a hydrophobic barrier that stops any water drops while allowing hydrocarbons to pass through. Horizontal configurations can be used for both water from hydrocarbon separations and hydrocarbon from water separations and rely on a settling zone to separate out the large coalesced drops from the bulk phase. Liquid–liquid coalescers can have a service life varying from 1 to 2 years when protected by prefiltration. For emulsions with interfacial tensions above 20 dyne/cm, glass fiber construction has proven sufficient, but for more difficult-to-separate emulsions with interfacial tensions <20 dyne/cm, polymeric construction will provide improved separation performance.

Typically, it will have good removal efficiency for fine drops of 1 μm and larger. The separation mechanisms are primarily by direct interception and attractive forces between the dispersed phase drops and the coalescer medium fibers (Van der Waals forces, ionic charge or any specific attraction based on the fiber material and drops). At reduced flow rates, the liquid–liquid coalescer will exhibit the same or improved separation efficiency.

1.3 Darcy’s law

One common starting point for modeling filtration is to start with the Darcy equation (Darcy, 1856; Bear, 1988; Ripperger et al., 2012; Rybakov and Semenova, 2017) which applies to flow through a porous media:

(1.1)

where: p=pressure drop, x=coordinate (m), vf=velocity (m/s), µ=dynamic viscosity [kg/(m·s)], k=permeability coefficient (m²)

A limitation for this equation is that it is constrained to low Reynolds numbers (laminar flow) with varying values of the upper Reynold’s number applicability reported from Re of 1 to 10 (Sobieski and Trykozko, 2014).

(1.2)

where: d=average particle diameter (in cake or bed), ρ=density of fluid (kg/m³)

Also the Darcy equation does not take into consideration interactions between the fluid/solid interfaces and the existence of no slip conditions at walls.

Darcy’s law alone, doesn’t take into account solids in the feed stream accumulating on the filter and that this can cause a separate porous layer or cake to form. The cake will change with time and can grow in thickness or can become compacted under pressure drop. Filters will furthermore become plugged with smaller particles or sticky material over time changing the porosity of the porous medium.

A number of modifications have been made to the basic Darcy equation. One modification to extend its range into higher flux rates where the Re>10 was made by Forchheimer in 1901 (Ward, 1964; Sarler et al., 2004; Sobieski and Trykozko, 2014; Sivanesapillai et al., 2014):

(1.3)

where: β=Forchheimer coefficient.here Forchheimer has added an additional term β · (ρ · vf²) with units of kinetic energy to address inertial effects.

A similar treatment of the higher Reynold’s number has been addressed by the Ergun equation (Prieur du Plessis and Woudberg, 2008).

Fortunately, most applications involving filtration are not at these higher Reynold’s numbers and mostly do involve laminar flow.

A modification of Darcy’s law to take into account the pore size and number of pores (void volume) has been addressed through the Kozeny-Carman equation (Carmon, 1937; Ripperger et al., 2012). Here the permeability coefficient is related to the porous structure properties as follows:

(1.4)

where αH is the specific filter resistance and can be approximated as follows:

(1.5)

with: ε=the void volume fraction (porosity), Sv=specific inner surface (of the filter medium or filter cake)

(1.6)

and ds=Sauter diameter, an average diameter of the particle sizes obtained by dividing the particle volume by the particle surface area

Modifying the Darcy equation with the Kozeny-Carman contribution gives:

(1.7)

This modeling does not take into account cake compression over time and fouling by smaller particles. Further discourses can be found in the literature on these subjects (Tiller and Cooper, 1960; Tiller and Yeh, 1986; Kim and Yuan, 2005; Ripperger et al., 2012; Eker et al., 2014; Bourcier et al., 2016).

An empirical modification to the Darcy equation to take into account viscous losses was suggested by Brinkman (1947) to give the following:

(1.8)

where µβ is the apparent Brinkman viscosity.

The term is intended to account for boundary conditions near walls and how the velocity profiles will be distorted. More advanced modeling using the modified Darcy-Brinkman equation has been used on arrays of small particles for different sized particles (Levy, 1983) and for the problem of flow in natural convection in porous media using a dual reciprocity boundary element method (Sarler et al., 2004).

For nonNewtonian fluids, the Darcy equation has been adjusted to take into account the effects of shear on viscosity (Khuzhayorov et al., 2000; Fadili et al., 2002).

More advanced modeling of many practical applications involves the combination of free flow section with that of a flow through a porous material section. The porous material/free flow sections may have fixed boundaries, or these boundaries can by dynamic. Examples of fixed boundaries include flow through porous rock formations, sand beds and cartridge filters. Dynamic boundaries are not set and vary over time due to changes in the system. Dynamic boundaries are found in crossflow filtration where concentration polarization exists, filtration with cake formation/use of filter aids and processes that involve formation of porous solids such as emulsion polymerization or metal deposition.

Previous work has coupled the Navier Stokes equations with Darcy’s law to model these complex flows followed by finite element analysis and use of computational fluid dynamics. For free flow, the Navier Stokes equation is used and for flow through porous material, the Darcy rule is followed. The boundary between the free flow and porous sections needs to allow for a coupling between the two regimes. One means to achieve this connection has been to define a slip boundary condition to match process variables including velocity, pressure, and shear at the interface introduced by Beavers and Joseph (1967).

Specific research related to the modeling of cartridge filters (Wakeman et al., 2005) to determine optimized pleat packing, flow through fibrous media (Nabovati et al., 2009), and crossflow (Pak et al., 2008; Hanspal